When putting pistons on rods...

You'll know if you're measuring everything and long before the heads go on. In some engines the rod side clearance is "used up" if the crank fillet is not adjacent to the rod's chamfer and either the crank won't turn (I always rotate the crank completely once after every rod get's torqued) or the rod side clearance will be non-existent or tight. Especially on strokers with performance cranks with big fillets the engine won't turn with two rods on a throw and one backwards.

And yes - I've got them backwards before...lol